rrdcached - Data caching daemon for rrdtool at Linux.org

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RRDCACHED(1) rrdtool RRDCACHED(1)

NAME
rrdcached - Data caching daemon for rrdtool

SYNOPSIS
rrdcached [-P permissions] [-l address] [-s group] [-w timeout] [-z delay] [-f timeout] [-p pid_file]
[-t write_threads] [-j journal_dir] [-F] [-g] [-b base_dir [-B]]

DESCRIPTION
rrdcached is a daemon that receives updates to existing RRD files, accumulates them and, if enough have been
received or a defined time has passed, writes the updates to the RRD file. A flush command may be used to
force writing of values to disk, so that graphing facilities and similar can work with up-to-date data.

The daemon was written with big setups in mind. Those setups usually run into IO related problems sooner or
later for reasons that are beyond the scope of this document. Check the wiki at the RRDtool homepage for
details. Also check "SECURITY CONSIDERATIONS" below before using this daemon! A detailed description of how
the daemon operates can be found in the "HOW IT WORKS" section below.

OPTIONS
-l address
Tells the daemon to bind to address and accept incoming connections on that socket. If address begins with
"unix:", everything following that prefix is interpreted as the path to a UNIX domain socket. Otherwise
the address or node name are resolved using "getaddrinfo()".

For network sockets, a port may be specified by using the form "[address]:port". If the address is an IPv4
address or a fully qualified domain name (i. e. the address contains at least one dot (".")), the square
brackets can be omitted, resulting in the (simpler) "address:port" pattern. The default port is 42217/tcp.
If you specify a network socket, it is mandatory to read the "SECURITY CONSIDERATIONS" section.

The following formats are accepted. Please note that the address of the UNIX domain socket must start with
a slash in the second case!

unix:</path/to/unix.sock>
/<path/to/unix.sock>
<hostname-or-ip>
[<hostname-or-ip>]:<port>
<hostname-or-ipv4>:<port>

If the -l option is not specified the default address, "unix:/tmp/rrdcached.sock", will be used.

-s group_name|gid
Set the group permissions of a UNIX domain socket. The option accepts either a numeric group id or group
name. That group will then have both read and write permissions (the socket will have file permissions
0750) for the socket and, therefore, is able to send commands to the daemon. This may be useful in cases
where you cannot easily run all RRD processes with the same user privileges (e.g. graph generating CGI
scripts that typically run in the permission context of the web server).

This option affects the following UNIX socket addresses (the following -l options) or the default socket
(if no -l options have been specified), i.e., you may specify different settings for different sockets.

The default is not to change ownership or permissions of the socket and, thus, use the system default.

-m mode
Set the file permissions of a UNIX domain socket. The option accepts an octal number representing the bit
pattern for the mode (see chmod(1) for details).

Please note that not all systems honor this setting. On Linux, read/write permissions are required to
The arguments given to the -P option is a comma separated list of commands. For example, to allow the
"FLUSH" and "PENDING" commands one could specify:

rrdcached -P FLUSH,PENDING $MORE_ARGUMENTS

The -P option affects the following socket addresses (the following -l options) or the default socket (if
no -l options have been specified). In the following example, only the IPv4 network socket (address
10.0.0.1) will be restricted to the "FLUSH" and "PENDING" commands:

rrdcached -l unix:/some/path -P FLUSH,PENDING -l 10.0.0.1

A complete list of available commands can be found in the section "Valid Commands" below. There are two
minor special exceptions:

· The "HELP" and "QUIT" commands are always allowed.

· If the "BATCH" command is accepted, the . command will automatically be accepted, too.

Please also read "SECURITY CONSIDERATIONS" below.

-w timeout
Data is written to disk every timeout seconds. If this option is not specified the default interval of
300 seconds will be used.

-z delay
If specified, rrdcached will delay writing of each RRD for a random number of seconds in the
range [0,delay). This will avoid too many writes being queued simultaneously. This value should be no
greater than the value specified in -w. By default, there is no delay.

-f timeout
Every timeout seconds the entire cache is searched for old values which are written to disk. This only
concerns files to which updates have stopped, so setting this to a high value, such as 3600 seconds, is
acceptable in most cases. This timeout defaults to 3600 seconds.

-p file
Sets the name and location of the PID-file. If not specified, the default,
"$localststedir/run/rrdcached.pid" will be used.

-t write_threads
Specifies the number of threads used for writing RRD files. The default is 4. Increasing this number
will allow rrdcached to have more simultaneous I/O requests into the kernel. This may allow the kernel to
re-order disk writes, resulting in better disk throughput.

-j dir
Write updates to a journal in dir. In the event of a program or system crash, this will allow the daemon
to write any updates that were pending at the time of the crash.

On startup, the daemon will check for journal files in this directory. If found, all updates therein will
be read into memory before the daemon starts accepting new connections.

The journal will be rotated with the same frequency as the flush timer given by -f.

When journaling is enabled, the daemon will use a fast shutdown procedure. Rather than flushing all files
to disk, it will make sure the journal is properly written and exit immediately. Although the RRD data
specified by a relative path, will be interpreted to be relative to this directory. If not given the
default, "/tmp", will be used.

+------------------------+------------------------+
! Command line ! File updated !
+------------------------+------------------------+
! foo.rrd ! /tmp/foo.rrd !
! foo/bar.rrd ! /tmp/foo/bar.rrd !
! /var/lib/rrd/foo.rrd ! /var/lib/rrd/foo.rrd !
+------------------------+------------------------+
Paths given on the command line and paths actually
updated by the daemon, assuming the base directory
"/tmp".

WARNING: The paths up to and including the base directory MUST NOT BE symbolic links. In other words, if
the base directory is specified as:

-b /base/dir/somewhere

... then NONE of the following should be symbolic links:

/base
/base/dir
/base/dir/somewhere

-B Only permit writes into the base directory specified in -b (and any sub-directories). This does NOT
detect symbolic links. Paths containing "../" will also be blocked.

AFFECTED RRDTOOL COMMANDS
The following commands may be made aware of the rrdcached using the command line argument --daemon or the
environment variable RRDCACHED_ADDRESS:

· dump

· fetch

· flush

· graph

· graphv

· info

· last

· lastupdate

· update

· xport

The update command can send values to the daemon instead of writing them to the disk itself. All other
commands can send a FLUSH command (see below) to the daemon before accessing the files, so they work with up-
be "now". The value and time of the value (called "Time" in the diagram below) are appended to the tree node.

When appending a value to a tree node, it is checked whether it's time to write the values to disk. Values are
written to disk if "now() - First >= timeout", where "timeout" is the timeout specified using the -w option,
see "OPTIONS". If the values are "old enough" they will be enqueued in the "update queue", i. e. they will be
appended to the linked list shown below. Because the tree nodes and the elements of the linked list are the
same data structures in memory, any update to a file that has already been enqueued will be written with the
next write to the RRD file, too.

A separate "update thread" constantly dequeues the first element in the update queue and writes all its values
to the appropriate file. So as long as the update queue is not empty files are written at the highest possible
rate.

Since the timeout of files is checked only when new values are added to the file, "dead" files, i. e. files
that are not updated anymore, would never be written to disk. Therefore, every now and then, controlled by the
-f option, the entire tree is walked and all "old" values are enqueued. Since this only affects "dead" files
and walking the tree is relatively expensive, you should set the "flush interval" to a reasonably high value.
The default is 3600 seconds (one hour).

The downside of caching values is that they won't show up in graphs generated from the RRD files. To get
around this, the daemon provides the "flush command" to flush specific files. This means that the file is
inserted at the head of the update queue or moved there if it is already enqueued. The flush command will
return only after the file's pending updates have been written to disk.

+------+ +------+ +------+
! head ! ! root ! ! tail !
+---+--+ +---+--+ +---+--+
! /\ !
! / \ !
! /\ /\ !
! /\/\ \ `----------------- ... --------, !
V / `-------, ! V
+---+----+---+ +------+-----+ +---+----+---+
! File: foo ! ! File: bar ! ! File: qux !
! First: 101 ! ! First: 119 ! ! First: 180 !
! Next:&bar -+--->! Next:&... -+---> ... --->! Next:NULL !
| Prev:NULL !<---+-Prev:&foo !<--- ... ----+-Prev: &... !
+============+ +============+ +============+
! Time: 100 ! ! Time: 120 ! ! Time: 180 !
! Value: 10 ! ! Value: 0.1 ! ! Value: 2,2 !
+------------+ +------------+ +------------+
! Time: 110 ! ! Time: 130 ! ! Time: 190 !
! Value: 26 ! ! Value: 0.1 ! ! Value: 7,3 !
+------------+ +------------+ +------------+
: : : : : :
+------------+ +------------+ +------------+
! Time: 230 ! ! Time: 250 ! ! Time: 310 !
! Value: 42 ! ! Value: 0.2 ! ! Value: 1,2 !
+------------+ +------------+ +------------+

The above diagram demonstrates:

· Files/values are stored in a (balanced) tree.

If your rrdtool installation was built without libwrap there is no form of authentication for clients
connecting to the rrdcache daemon!

If your rrdtool installation was built with libwrap then you can use hosts_access to restrict client access to
the rrdcache daemon (rrdcached). For more information on how to use hosts_access to restrict access to the
rrdcache daemon you should read the hosts_access(5) man pages.

It is still highly recommended to install a packet filter or similar mechanism to prevent unauthorized
connections. Unless you have a dedicated VLAN or VPN for this, using network sockets is probably a bad idea!

Authorization
There is minimal per-socket authorization.

Authorization is currently done on a per-socket basis. That means each socket has a list of commands it will
accept and it will accept. It will accept only those commands explicitly listed but it will (currently) accept
these commands from anyone reaching the socket.

If the networking sockets are to be used, it is necessary to restrict the accepted commands to those needed by
external clients. If, for example, external clients want to draw graphs of the cached data, they should only
be allowed to use the "FLUSH" command.

Encryption
There is no encryption.

Again, this may be added in the future, but for the time being it is your job to keep your private data
private. Install a VPN or an encrypted tunnel if you statistics are confidential!

Sanity checking
There is no sanity checking.

The daemon will blindly write to any file it gets told, so you really should create a separate user just for
this daemon. Also it does not do any sanity checks, so if it gets told to write values for a time far in the
future, your files will be messed up good!

Conclusion
· Security is the job of the administrator.

· We recommend to allow write access via UNIX domain sockets only.

· You have been warned.

PROTOCOL
The daemon communicates with clients using a line based ASCII protocol which is easy to read and easy to type.
This makes it easy for scripts to implement the protocol and possible for users to use telnet to connect to
the daemon and test stuff "by hand".

The protocol is line based, this means that each record consists of one or more lines. A line is terminated by
the line feed character 0x0A, commonly written as "\n". In the examples below, this character will be written
as "<LF>" ("line feed").

After the connection has been established, the client is expected to send a "command". A command consists of
the command keyword, possibly some arguments, and a terminating newline character. For a list of commands, see
"Valid Commands" below.

2 Two lines follow<LF>
This is the first line<LF>
And this is the second line<LF>

Valid Commands
The following commands are understood by the daemon:

FLUSH filename
Causes the daemon to put filename to the head of the update queue (possibly moving it there if the node is
already enqueued). The answer will be sent after the node has been dequeued.

FLUSHALL
Causes the daemon to start flushing ALL pending values to disk. This returns immediately, even though the
writes may take a long time.

PENDING filename
Shows any "pending" updates for a file, in order. The updates shown have not yet been written to the
underlying RRD file.

FORGET filename
Removes filename from the cache. Any pending updates WILL BE LOST.

QUEUE
Shows the files that are on the output queue. Returns zero or more lines in the following format, where
<num_vals> is the number of values to be written for the <file>:

<num_vals> <file>

HELP [command]
Returns a short usage message. If no command is given, or command is HELP, a list of commands supported by
the daemon is returned. Otherwise a short description, possibly containing a pointer to a manual page, is
returned. Obviously, this is meant for interactive usage and the format in which the commands and usage
summaries are returned is not well defined.

STATS
Returns a list of metrics which can be used to measure the daemons performance and check its status. For a
description of the values returned, see "Performance Values" below.

The format in which the values are returned is similar to many other line based protocols: Each value is
printed on a separate line, each consisting of the name of the value, a colon, one or more spaces and the
actual value.

Example:

9 Statistics follow
QueueLength: 0
UpdatesReceived: 30
FlushesReceived: 2
UpdatesWritten: 13
DataSetsWritten: 390
TreeNodesNumber: 13
TreeDepth: 4
JournalBytes: 190
JournalRotate: 0

BATCH
This command initiates the bulk load of multiple commands. This is designed for installations with
extremely high update rates, since it permits more than one command to be issued per read() and write().

All commands are executed just as they would be if given individually, except for output to the user.
Messages indicating success are suppressed, and error messages are delayed until the client is finished.

Command processing is finished when the client sends a dot (".") on its own line. After the client has
finished, the server responds with an error count and the list of error messages (if any). Each error
messages indicates the number of the command to which it corresponds, and the error message itself. The
first user command after BATCH is command number one.

client: BATCH
server: 0 Go ahead. End with dot '.' on its own line.
client: UPDATE x.rrd 1223661439:1:2:3 <--- command #1
client: UPDATE y.rrd 1223661440:3:4:5 <--- command #2
client: and so on...
client: .
server: 2 Errors
server: 1 message for command 1
server: 12 message for command 12

QUIT
Disconnect from rrdcached.

Performance Values
The following counters are returned by the STATS command:

QueueLength (unsigned 64bit integer)
Number of nodes currently enqueued in the update queue.

UpdatesReceived (unsigned 64bit integer)
Number of UPDATE commands received.

FlushesReceived (unsigned 64bit integer)
Number of FLUSH commands received.

UpdatesWritten (unsigned 64bit integer)
Total number of updates, i. e. calls to "rrd_update_r", since the daemon was started.

DataSetsWritten (unsigned 64bit integer)
Total number of "data sets" written to disk since the daemon was started. A data set is one or more values
passed to the UPDATE command. For example: "1223661439:123:456" is one data set with two values. The term
"data set" is used to prevent confusion whether individual values or groups of values are counted.

TreeNodesNumber (unsigned 64bit integer)
Number of nodes in the cache.

TreeDepth (unsigned 64bit integer)
Depth of the tree used for fast key lookup.

JournalBytes (unsigned 64bit integer)
Total number of bytes written to the journal since startup.

SIGUSR2
The daemon exits immediately, without flushing updates out to disk. Pending updates will be replayed from
the journal when the daemon starts up again. WARNING: if journaling (-j) is NOT enabled, any pending
updates WILL BE LOST.

BUGS
No known bugs at the moment.